1. Field of the Invention
This invention relates to diagnosing internal combustion engines electronically.
2. Description of the Prior Art
In internal combustion engines, proper operation is related to delivery of fuel at proper pressures as a function of engine speed, and as a function of intake manifold pressure. Particularly in the case of diesel engines, the speed of the engine may be controlled by fuel pressure, and overfueling of the engine, which can result in excessive smoke and possible engine damage, is prevented by reducing fuel pressure at the inlet to the engine whenever an inadequate air intake pressure exists.
In diesel engines, it has long been known to check fuel pumps against certain operating standards, particularly to check the pressure of fuel delivered by the fuel pump at the fuel inlet rail for various speeds of the engine (which are herein referred to as the first and second checkpoints and the rated point) and the point at which the governor limits the fuel pressure to avoid excessive engine speed (called cut-off). However, in the prior art it has been common to remove the fuel pump from the engine and mount it on a specially designed test rig in order to operate the pump at desired speeds in a steady state condition, and monitor the pressure developed by the pump at those desired speeds. This in turn necessitates an excessive amount of labor in removing the fuel pump from the engine, as well as incurring the potential for inadvertently causing additional problems as a result of the mechanical steps involved in the pump removal and reinstallation procedures.
A seemingly small, but practically troublesome aspect in analyzing fuel system is the point at which rail fuel pressure is to be measured. If the pressure measurement is to reflect the pressure of the fuel pump itself, it must be in full fluid communication therewith; this is particularly true where the fuel pressure is to be measured under high fueldelivery conditions. On many engines, it is extremely difficult and impractical to measure fuel pressure upstream of the throttle. In order to provide adequate pressure measurements, the throttle must therefore be fully open for on-engine fuel diagnostics. Also, the pump should be tested under full flow conditions. Obviously, a vehicle-mounted engine responding to a fully open throttle will simply advance from low idle to governor-controlled cut-off speed, at a very rapid rate, which renders the measurement of fuel pressures at designated, specification speeds impossible. To overcome this, it has been known to make open-throttle, steady speed fuel pressure measurements of an engine mounted on a vehicle which is standing on a dynomometer. As is known, the dynomometer rotates with the power driving wheels of the vehicle, and can be loaded in a controlled fashion from no load (simulating a slight downgrade) to full load (simulating a fully loaded vehicle climbing a hill). Thus, the dynomometer load can be adjusted to hold the desired specification speed under full power, with the throttle wide-open. However, a dynomometer is a very cumbersome and expensive test stand, and is frequently totally unavailable where vehicle or engine diagnosis is required.
There are other examples of the need to perform open throttle tests at specified speeds. An aneroid fuel pressure control is normally tested at at least one speed to determine the manner in which fuel pressure varies with speed when the pressure is also controlled by the aneroid. However, as in the case of fuel pressure tests relating to the fuel pump and other portions of the fuel system, if on-engine testing is to be performed, it must be done with a full throttle in order to permit measuring the fuel pressure, and without a dynomometer to load the engine, full throttle will cause rapid acceleration from idle to governor cut-off, with no ability to carefully limit operation to a desired speed. A further example is in the testing of the fuel inlet pressure (the pressure at the upstream end of the fuel pump, downstream from a fuel filter), the measure of which compared against ambient pressure is an indication of the degree of restriction. But the degree of restriction as indicated by pressure is also a function of the amount of fuel flowing; therefore full flow pressure is required as a test of the fuel inlet restriction. In order to achieve full fuel flow, the throttle must be open; and, without a dynomometer, the apparatus will simply accelerate to governor cut-off speed at which point the governor will reduce the fuel pressure to a very low amount, thereby rendering the inlet fuel restriction test impossible to perform.